This invention relates to bottom-blown metallurgical processes.
One iron refining method known as Q-BOP involves the injection of oxygen into a molten metal bath through tuyeres located beneath the metal surface. In order to minimize or control the erosion of tuyeres and the adjacent refractory, a hydrocarbon shielding fluid, such as propane, natural gas, coal gas or light oil is commonly injected in surrounding relation to the oxygen stream. Fluxing agents such as burnt lime may also be injected into the vessel by entrainment in oxygen or in nonoxidizing gases. This process is described in U.S. Pat. No. 3,706,549.
Because the oxidation of such elements as carbon, silicon, phosphorus and manganese, in ferrous metal is exothermic, it is possible in the Q-BOP process to at least partially employ solid ferrous materials such as scrap, iron pellets, solid pig iron and the like which are generally more economical than hot metal. In order to increase the ability to utilize increased proportions of solid iron in the Q-BOP process, additional outside sources of energy have been employed. One such method is discussed in U.S. Pat. No. 4,198,230 wherein the concentric two-pipe tuyeres are employed as preheat burners.
During the normal oxygen blow in Q-BOP vessels, the amount of hydrocarbon required for tuyere and refractory protection is only a small proportion by volume of the oxygen which is introduced for the purpose of reducing carbon and other elements. For example, when propane is employed as the shielding fluid, its flow rate is only about one percent by volume of that of oxygen during the refining stage of the process. As a result, the center tuyere pipe is substantially larger in area than the gap between the center and outer tuyere pipes. This is satisfactory, of course, when the tuyeres are used for oxygen injection. However, when the tuyeres are used as preheat burners, it is not possible to obtain stoichiometric conditions when gaseous fuels are employed. Accordingly, in the process of U.S. Pat. No. 4,198,230, a denser hydrocarbon, such as oil, is employed as the preheat fuel. For this reason, it is necessary to provide switching valves which permit delivery of oil to the gap between the tuyere pipes during the preheat period and a gaseous hydrocarbon source during the main oxygen blow.
In order to further increase the hydrocarbon fuel available for preheating and scrap melting, it has also been suggested that a solid fuel, such as coke, be employed. This material may be added along with the solid metallic charge or it may be entrained in a nonoxydizing gas, such as nitrogen, and injected through at least some of the tuyeres. The use of solid fuels also requires switching prior to the main oxygen blow or refining step.